People speak easily about the poetry of theatre, as if it were self evident, as if it were merely an ornament available to those who might choose to employ it. But poetry is not an easy thing. It is the pith and passion of plays, the molten spine of them; it is the profundity that is summoned by the carnality of language, the mystery of the corporeal and the mortal: this particular body in this specific time, speaking what cannot be repeated.

The Black Swan of Trespass and Stalking Matilda are very different plays; but both have been described as "poetic". It must be said that the poetry of theatre differs, markedly and importantly, from poems: but it is also related, in ways which are not necessarily obvious but which remain profound. Predictably I suppose, given my own predelictions, what struck me in both productions was a conviction that theatre practitioners would benefit from a better understanding of poetry.

The Black Swan of Trespass concerns itself with one of the most celebrated hoaxes in Australian literary history. In 1943 Harold Stewart and James McAuley, two poets with a particular animus towards the modernist work of writers like Dylan Thomas and Henry Treece, cooked up a fictional poet called Ern Malley. They created his life's work (sixteen poems called The Darkening Ecliptic) in an afternoon's hijinks of creative collage using, among other things, a Complete Works of Shakespeare and an army training manual on mosquitos. They concocted a letter from Ern's sister Ethel that described his life as a garage mechanic and his tragic early death from Graves disease, and sent the lot to Max Harris, then the young, iconoclastic editor of the modernist journal Angry Penguins.

As is well known, Harris enthusiastically published the poems, proclaiming Malley a genius. When Stewart and McAuley exposed the hoax, he stuck to his guns; whether or not they had intended it, he said, the poems were still extraordinary. But the story took another twist when Harris was prosecuted for obscenity in a courtcase which has shades of Pythonesque black comedy. The ultimate irony is that the poems have passages of undeniable beauty, and are now probably the most famous pieces of writing either Stewart or McAuley published. Malley generated a compelling reality: there is even a celebrated portrait of him by Sidney Nolan.

In The Black Swan of Trespass, writer Lally Katz and director Chris Kohn conjure some charming theatre from the ghostly figure of the imaginary poet. Ern Malley is summoned by Stewart and McAuley, who are represented by comically grotesque puppets - a chicken and a cat - on either side of the stage, and Ern himself (Christopher Brown) stands before us, tall, rangy, surreally Australian, all his suburban pathos framed in the velvet curtains of a puppet theatre.

The irony of Malley's situation as a poet who does not exist is not lost on him. As a theatrical creation, he is uneasily aware, as in fact any conscious writer must be, that his language is at best only partly his own and may be, in fact, writing him, that his writerly self is a fiction that trespasses hesitantly on the "alien waters" of reality. As the poet says in Dürer: Innsbruck, 1495, the work from which the show takes its title and which is to my mind the loveliest of the Malley poems:

"I had read in books that art is not easy But no one warned that the mind repeats In its ignorance the vision of others. I am still The black swan of trespass on alien waters."

The theatrical realisation of these complexities is often enchanting. Chris Kohn employs music, stylised performance and projected text as well as an ingeniously surreal design to create a show that works on multiple levels, and which seeks to express the pathos and irony of both Malley's unstable existence and his writings. Characters which are imaginary even in Ern's reality - Anopholes (Gavan O'Leary), a kind of mosquito-muse/narrator, and Princess (Jacklyn Bassanelli), his Keatsian love object - thicken the texture further. There are moments in this show - most often when Malley says his own poems - when all these complexities fold together into a shimmering, vital present.

One such scene is when Ethel (Katie Keadie) and Ern venture out into the real world, and speak about the people hurrying home - a description reminiscent of John Brack's painting Collins St, 5pm. Here the force of their yearning to be like other people, to be real, attains a potent and fragile poignancy. But too often this delicacy and poise is blurred in Lally Katz's script, which cannot but suffer by comparison to the complexity of the Malley poems: sometimes merely simplistic in its responses to them, it often veers dangerously close to sentimentalisation.

In the final scene, where Ern speaks his Petit Testament, I writhed at the bad judgement of punctuating each verse with lyrics from a sentimental love song. It was as if the dislocations of the poem, the contradictions of the poet and the poetry itself, could be resolved through a comfortingly simple narrative of unattainable and tragic love. No, it's not that simple.

Tee O'Neill's Stalking Matilda, directed by Chris Bendall for Theatre@Risk, addresses a more familiar theatrical poetic, that of the chorus. But this chorus is not a formal Greek convocation of witnesses, but something more like Brecht's idea of epic theatre in his essay The Street Scene, in which members of the cast enact the events they are describing, fluidly moving in and out of character.

Stalking Matilda was originally commissioned in Ireland, and has apparently been rewritten to reflect "an Australian setting and spirit". Perhaps its first problem is that, despite its concern with the plight of asylum seekers, common to both countries, it does not easily make the transition from Ireland to Australia. Local conditions, I couldn't help reflecting as I watched, do count; the English/Irish "hoodies" are not the same as Australian gangs; we have not had a celebrated racist murder like the Stephen Lawrence case in England (obliquely referred to in the play); and racism here is, if equally ugly, different in its ugliness.

The major difference between Ireland and Australia is perhaps that Australian history since settlement has been characterised by successive waves of immigrants and so is, whether we admit it or not, deeply multicultural, whereas Ireland is a racially homogenous society marked more by emigration than immigration, in which the sudden intrusion of "aliens" registers as a shock. The notion of "aliens" - central to the metaphor of the play - is somewhat muffled here by the immigrant status of so many of us. This uneasiness of locale undermines the play's potency; it might have been better to present it in its full Irishness.

Despite this, there is much to like in O'Neill's writing, in particular her robust embrace of human complexity and her refusal of easy moralising. The dilemmas faced by asylum seekers are sketched briskly and without sentiment. O'Neill portrays the injustices they face, desperately fleeing their own countries only to become persecuted non-citizens in a country that does not want them, but she also reveals the resilience and comic subversiveness of the oppressed, the small but vital ways in which human beings can help each other survive.

Central to the play is the figure of Matilda (Jude Beaumont), a charismatic, beautiful woman whose decision to walk into the sea and disappear is the mystery that sparks the action. The chorus - Irene Dios, Odette Joannidis, Ron Jordan, Toby Newton and Jeremy Stanford - discover her mobile phone on the beach and read a series of text messages, which reveal complexities, hypocrisies, strange elisions. The play is structured around a gradual enactment - not necessarily in chronological order - of the events that led to those cryptic notes.

Matilda is in many ways a symbol of European liberal ambivalence and its inevitable complicity with power. On the one hand, she is devoted to helping her asylum seeker friends, getting them false documents, spending hours with them in the bureaucratic maze of applying for citizenship, celebrating with them their small triumphs, even marrying Suleyman (Rob Jordan), a refugee from an unnamed African country. (Although ethnicities - Eastern European, African - are broadly suggested, countries are referred to by number - "First World", "Sixteenth World" - which is an economical and effective distancing device).

On the other, Matilda has a perverse affair with the General (Jeremy Stanford), a shadowy and sinister military commander who puts money into her bank account after every sexual encounter. And she may be also responsible for the death of Suleyman, who appears to be the victim of a brutal race hate crime, and for the burning down of the boarding house where her immigrant friends live.

The chorus sets off to discover the truth of these contradictory signs, but the murder mystery impulse isn't enough to sustain the energy of the play, despite a gallery of skilfully drawn characters and some interesting scenes. On the night I saw it, the action flagged considerably in its second half; it was a bit of a race to see whether the play would end before I lost interest entirely in why Matilda had walked into the sea. Jude Beaumont turning in anguish to the waves became, in truth, a rather over-used image during the course of the show. Though, in mitigation, I note that the given running time is 90 minutes, and the show I saw went for two hours. It could have been an exceptionally slow night that exaggerated the longeuers and repetitions in the production.

But there were other problems that were not to do with pacing. Especially towards the end, the chorus was often written with a self-conscious, literal clumsiness that slowed down the play. The physical-theatre aspect of the performances, together with the day-glo circus set, at times reminded me of those breathlessly earnest Theatre In Education shows English teachers used to inflict on unwitting adolescents in the name of culture. At its best, the production transcended these associations, but not often enough to lift the show out of its problems.

There is a poetic of a promising muscularity at work in this play, but it stumbles. Poetry exists in the silences between words, in what is not said, at least as much as it does in the words spoken; and sometimes in Stalking Matilda there were just too many notes.